Diesel engines are powerplants of many trucks that are presently being manufactured in North America.
Diesel engine combustion must be precisely controlled to minimize emissions. Among the factors in determining emission levels is the presence of excess oxygen in the combustion process. Exhaust gas recirculation (EGR) is the process of recirculating a portion of an engine's exhaust back into the engine's cylinders, and it has been used to reduce peak combustion temperatures, lower excess oxygen levels, and reduce NOx emissions.
Typically, diesel engines are equipped with single or two-stage turbochargers. A two-stage turbocharger comprises high- and low-pressure turbines in series flow relationship in the exhaust system that operate high- and low-pressure compressors in series flow relationship in the intake system to develop boost is one example of a turbocharger. A single-stage stage turbocharger has only a single turbine and a single compressor.
U.S. published application 2008/0078176 describes an engine control system for a turbocharged diesel engine wherein, when actual boost deviates from a desired boost set-point developed by a boost control strategy, such as during a sudden acceleration or deceleration, the strategy provides a prompt adjustment of exhaust gas recirculation (EGR) seeking to null out the boost disparity.
U.S. published application 2008/0078176 describes a strategy for control of an EGR valve that establishes a desired EGR set-point based on several parameters, including engine speed, indicated engine torque, and mass flow rate of fresh air entering the intake system. The EGR valve is controlled by a duty-cycle signal that is based on the EGR set-point. Changes in the EGR set-point change the duty cycle of the duty signal through a controller, typically a PID (proportional-integral-derivative) controller embodied as a virtual controller in the processing strategy.
The strategy of U.S. published application 2008/0078176 for control of turbocharger boost establishes a desired boost set-point based on several parameters, including engine speed and indicated engine torque. The boost set-point is processed by a control strategy for controlling the turbocharger, specifically controlling the position of the vanes of a variable geometry turbocharger (VGT). Vane position is typically controlled by an actuator to which a duty-cycle signal based on boost set-point is applied. The duty-cycle signal may also be developed by a PID controller in the boost control strategy.
In accordance with U.S. published application 2008/0078176, various calculations are made. One calculation performed by a suitably appropriate algorithm uses actual boost to provide the mass flow rate through the engine cylinders. Another calculation, performed in any suitably appropriate way, provides the actual mass flow rate of fresh air entering the engine intake system. The mass flow rate of recirculated exhaust gas that mixes with the fresh air entering the intake system is then calculated as the difference between the calculated mass flow rate through the engine cylinders and the actual mass flow rate of fresh air entering the intake system.
The EGR valve is modeled in such a way that for certain prevailing conditions that bear on mass flow rate through the EGR valve, such as exhaust gas temperature and pressure differential between the valve inlet and outlet, a correlation between mass flow rate through the valve and the extent to which the EGR valve is open is defined.
To null out the boost disparity during a sudden acceleration or deceleration, the control system uses the correlation between flow rate through the EGR valve and the extent to which the EGR valve is open to define an adjustment for the valve opening that will adjust the mass flow through the EGR valve in a way that seeks to null out the boost discrepancy.
The control system of U.S. published application 2008/0078176 performs feed-forward adjustment of the mass flow rate of recirculated exhaust gas in a direction of adjustment that seeks to null out the difference between desired boost set point and actual boost. The system processes the data representing the difference between the data representing actual mass flow rate through the engine cylinders and the data representing the expected mass flow rate through the engine cylinders to develop a feed-forward adjustment signal that is applied to the EGR valve to cause the adjustment.
The above described method controls the ratio of EGR gas to fresh air that can be disrupted during transient conditions such as during vehicle acceleration and deceleration.
The present inventor recognizes that in reality, EGR contains a variable amount of oxygen, somewhere from 0% to 21%. With an increased demand for engine improvements in reduced emissions, the assumption that EGR does not contain oxygen presents a limitation for further reducing emissions.
The present inventor recognizes the need for an engine control system for a diesel engine that more closely controls combustion and fuel/air mixtures to provide for reduced emissions and enhanced engine efficiency. The present inventor recognizes the need for an excess oxygen control algorithm that attempts to accurately monitor and precisely control excess oxygen available for combustion in a diesel engine.